Over 200,000 women are expected to be diagnosed with breast cancer in 2010, and approximately half will receive radiotherapy to prolong survival and improve localized tumor control. The vast majority of breast cancer radiotherapy patients (74%-100%) will experience skin toxicity during treatment. The physical manifestations of skin toxicity range from erythema (redness, warmth, rash-like appearance), to dry desquamation (dryness, itching, peeling), moist desquamation (oozing, tender, redness and exposure of the dermis), and necrosis. Severe skin toxicity can even necessitate treatment breaks, which can reduce local disease control and increase overall mortality. Skin toxicity can affect multiple aspects of quality of life (QOL) including sensation (pain, itching, burning), body image, emotional well-being, lifestyle, and treatment satisfaction. Despite the prevalence of skin toxicity, there is no gold standard measure of skin toxicity-related quality of life (STR-QOL). The lack of a measure of STR-QOL is a serious concern for at least three reasons: 1) without a sound assessment tool, important clinical aspects of patients' experiences may go unrecognized and unmanaged; 2) breast cancer patients cannot make fully informed treatment decisions without accurate information on radiotherapy side effects and effects on QOL; and 3) a reliable, valid, and responsive scale is needed in order to rigorously evaluate new strategies and interventions to prevent and control skin toxicity. Without such a scale, the efficacy of skin toxicity prevention and control efforts will be difficult, if not impossible, to interpret. Therefore, consistent with the NIH Roadmap's recognition of the pressing need to better quantify clinically important symptoms and outcomes, the goal of the project is to produce the first psychometrically sound and clinically meaningful scale to assess STR-QOL in breast cancer radiotherapy patients - the Profile of Skin Toxicity-Breast (POST-B). The first specific aim of the project is to assemble the POST-B from the building blocks of our preliminary item pool. Using state-of-the-art psychometric approaches (item response theory), we will reduce the 111 preliminary items to yield a final, concise, psychometrically sound scale that maximizes information gained while minimizing patient burden. The second specific aim is to determine the reliability and validity of the POST-B. The third specific aim is to determine the responsiveness (sensitivity to change) of the POST-B. Results of the proposed study have the potential to increase our understanding of the patient experience of skin toxicity, to improve symptom assessment and management in clinical settings, and to provide a much needed outcome measure for clinical trials of skin toxicity prevention and control efforts.